1. Field of the Invention
The present invention relates to automatic calibration methods, read apparatuses and storage apparatuses, and more particularly to an automatic calibration method and a read apparatus adapted for a magnetic disk drive using a magneto-resistive (MR) head and characterized by a variation in the environment in which a reading operation takes place, and a storage apparatus using a demodulating system in such a read apparatus.
Recently, the speed of computer operation has become increasingly high and the size of a computer unit has become increasingly small. A direction for high-speed and size reduction is also required for a magnetic disk drive which is used as an external storage unit. In order to realize high-speed and size reduction of a magnetic disk drive, it is necessary to conduct a high-density recording on a magnetic disk. Hence, a high density of data bits results.
In a data demodulating method using a peak detection, a point of variation in data is detected by differentiating a peak point in a waveform of restored data so as to detect a zero crossing point. For this reason, a large peak shift is invited in the conventional data demodulating method when the density of data bits becomes high, due to interwaveform interference (intersymbol interference). A large peak detection error is thus created. As a result, the error rate of the magnetic disk drive increases and the reliability suffers.
For the reason stated above, there is a limit to the effort of improving the reliability of the magnetic disk drive if the conventional data demodulating method is employed in high-density recording. One approach for resolving this situation is a data demodulating method wherein a partial response method called a PR4ML (partial response class 4 maximum likelihood) method is employed. In the PR4ML method, interwaveform interference is assumed as a precondition and taken advantage of.
2. Description of the Related Art
In a demodulating system within a read/write circuit of a conventional magnetic disk drive, parameters of various circuits constituting the demodulating system are predetermined. Specifically, parameters of various circuits in the demodulating system are calibrated under a certain condition (for example, under a normal-temperature condition) and stored in a memory as default values for each cylinder zone. Each time a switching between cylinder zones takes place, default values for the target cylinder zone are read from the memory and set in the circuits in the demodulating system. Thereby, a variation in the characteristic of the demodulating system due to the switching between cylinder zones is canceled.
Generally, the conventional magnetic disk drive uses an inductance head and uses a peak detection method as a data demodulating method. In the demodulating system, the level of equalization by a cosine equalizer is fixed for each cylinder zone. A variation in the characteristic of the demodulating system due to a variation in the characteristic of a head or due to a variation in the temperature is canceled by a margin (read margin) provided by a read characteristic of the demodulating system.
FIG. 1 is a block diagram showing an example of the conventional magnetic disk drive using the peak detection method. Referring to FIG. 1, NRZ (non-return-to-zero) data transferred from a host system (not shown) via an interface 111 is converted into serial data by a serial/parallel conversion circuit 110 and encoded by a RLL (run length limited) encoder circuit 117. The encoded data is fed to a write precompensation (hereinafter, referred to as write precomp.) circuit 119 for setting the level of write precompensation and fed to a write flip-flop (FF) 120. An output of the write FF is applied to a head 101 via a driver 121 and written on a magnetic disk 102.
The data written on the magnetic disk 102 is read by the head 101 and fed to a low-pass filter 106 via a fixed gain amplifier 104 and a variable gain amplifier 105. A gain of the variable gain amplifier 105 is automatically set by an automatic gain control (AGC) circuit 113 based on an output from the low-pass filter 106. A memory 116 stores parameters of a cosine equalizer circuit 107. A microprocessor unit (MPU) 115 sets parameters stored in the memory 116 in the cosine equalizer circuit 107. Consequently, a restored waveform output by the low-pass filter 106 is fed to a differential zero crossing detection circuit 108 after being subjected to equalization by the cosine equalizer circuit 107.
The differential zero crossing detection circuit 108 detects a point of variation in data by differentiating a peak point of the restored data waveform so as to detect a zero crossing point. An output of the differential zero crossing detection circuit 108 is fed to an RLL decoder circuit 109 via a VFO (variable frequency oscillator) circuit 114. The RLL decoder circuit 109 decodes an output of the VFO circuit 114. The decoded data is converted into parallel data (NRZ data) by the serial/parallel conversion circuit 110 and transferred to the host system via the interface 111.
However, if the parameters of the various circuits in the demodulating system are fixed for each cylinder in the high-density recording on the magnetic disk, the effect caused by a variation in the characteristic of a head or a variation in the temperature cannot be canceled by a margin of the read characteristic of the demodulating system. As a result, the error rate of the magnetic disk drive increases and the reliability suffers.
It is particularly to be noted that an MR head produces a relatively large variation in the characteristic of the demodulating system in response to a variation in the environment (for example, the temperature) in which the MR head is used. Hence, if the parameters of the various circuits in the demodulating system employing the PR4ML method are fixed for each cylinder, the effect caused by the variation in the characteristic cannot be canceled by a margin of the read characteristic of the demodulating system. As a result, the error rate of the magnetic disk drive increases and the reliability suffers.